Canadian Patent No. 1,168,006 (Wrasidlo) discloses an asymmetric ultrafiltration membrane having a skin layer supported by an "open honeycomb structure". The specification states that only polymeric materials having glass transition temperatures of at least 200.degree. C. would be suitable in the invention since only polymers having such relatively high glass transition temperatures are sufficiently rigid to form the honeycomb support structure. Of the polyamides, only polyarylamides were specifically identified as useful. The specification also states that the skin layer of the membranes has slit-like fissures instead of substantially circular pores.
U.S. Pat. No. 4,629,563 (Wrasidlo) discloses asymmetric membranes which may be used as ultrafilters or microfilters. The membranes can have a skin layer and a porous, asymmetric, reticulated support layer. The membranes described in the '563 patent can be made of polyamide, however, only polyhexamethylene terephthalamide, an araliphatic polyamide, is specifically disclosed. The membranes are formed by casting a polymer dope while that dope is in an unstable liquid dispersion condition.
U.S. Pat. No. 4,627,992 (Badenhop et al.) describes membranes formed from aromatic polyamide resins. The membranes described in the '992 patent can be symmetric or asymmetric and can be produced using a solution of the polyamide in an aprotic solvent.
U.S. Pat. No. 4,340,481 (Mishiro et al) describes a hollow fiber membrane having a three-dimensional net-like structure of fine pore passages. In Example 8, one such hollow fiber membrane is prepared from a casting solution containing 18 weight percent polyamide, methyl alcohol, water and calcium chloride dihydrate using a 1:1 methanol/water coagulation bath.
U.S. Pat. No. 4,722,795 (Gohl et al.) describes asymmetric, self-supporting membranes, useful for ultrafiltration, which may be in the form of a flat sheet, tube, or hollow fiber. The patent states that suitable membrane materials are polymers which are soluble in polar, non-protonic organic solvents such as dimethylsulfoxide. While the patent states that polyamides are preferred for use in forming the membranes, only an aromatic polyamide is specifically disclosed.
U.S. Pat. No. 3,876,738 (Marinaccio et al.) describes a process for making microporous membranes in which a film-forming polymer, e.g. nylon, is dissolved in a solvent system, e.g., a formic acid solution, the resulting solution is cast to form a film, and the film is quenched in a non-solvent bath, e.g. water/salt or alcohol/salt solutions, to form a microporous membrane. This patent does not state that these membranes are asymmetric.
U.S. Pat. No. 4,482,514 (Schindler et al.) describes a process for preparing ultrafiltration membranes using a formic acid solution of polyamide containing about 1 to 7% polyethylene glycol. The patent states that the membrane possesses an ultrafiltration skin layer and a backing layer in which the pore size increases with the distance from the ultrafiltration skin. FIG. 2 of the patent depicts a cross-sectional view of a membrane having a gradually increasing pore size.
Japanese Kokai Application JP 58094863, laid open June 6, 1983, describes polyamide membranes having a rough surface and a smooth surface. It describes the process of preparing one such membrane which includes preparation of a 10 to 40 weight percent solution of nylon 6,6 dissolved in formic acid mixed with calcium chloride and water, casting of the resulting solution, and intermittent coagulation, e.g. by repeated dipping of the cast membrane in a coagulation bath, of the cast film to form an uneven surface. The disclosed pore sizes range from 0.05 to 500 .mu.m on the rough surface and 1 to 1000 .mu.m on the smooth surface.
U.S. Pat. No. 3,615,024 (Michaels) describes anisotropic membranes which can be formed from polyamides such as polyhexamethylene adipamide and other such polyamides known as "nylon", however, no such membranes are exemplified.
U.S. Pat. Nos. 4,340,480 and 4,340,479 (Pall et al.) disclose a process for preparing skinless liquofilic alcohol-insoluble polyamide microfiltration membranes from polyamide having a ratio of methylene to amide groups of about 7 to 1 to about 12 to 1. The patents disclose that membranes prepared according to the described process have pores of uniform diameter. The patents also state that their membranes can have "tapering pores" but they do not describe highly asymmetric membranes.